1. Field of the Invention
The present invention relates to an engine unit of a motorcycle, which includes a transmission, a clutch mechanism, a starter motor, and accessories or auxiliary equipments such as a generator and the like.
2. Related Art
Regarding an engine unit of a motorcycle, for example, Japanese Patent No. 2841649 discloses an engine unit including: a crank shaft and a counter shaft, which are journaled in parallel with each other in an engine case; a primary drive gear mounted on the crank shaft; a primary driven gear mounted on the counter shaft and meshed with the primary drive gear; primary dampers provided to the primary driven gear for absorbing rotational shocks; a clutch mechanism provided to the primary driven gear for transmitting and interrupting the rotation of the primary driven gear with respect to the counter shaft; and an accessory drive gear disposed juxtaposedly with the primary driven gear which drives an accessory and through which the driving force of a starter motor is transmitted to the crank shaft.
As mentioned, by providing the primary dampers to the primary driven gear , it makes possible to absorb torque variations (rotational shocks) of the crank shaft at the startup of the engine by the primary dampers and to prevent the rotational shocks from being transmitted to the counter shaft and the downstream side thereof in the power transmission path. This leads to an improvement in the ride comfort of a motorcycle and a reduction in gear noises due to backlashes between gears, i.e., an improvement in quietness.
However, in the arrangements of the conventional structure mentioned above, since the accessory drive gear disposed juxtaposedly, i.e., in parallel, with the primary driven gear is rotated integrally with the primary driven gear, the accessory drive gear is disposed upstream of the primary dampers in the engine power transmission path. As a result, rotational shocks due to the torque variations and the like are transmitted to the accessory side through the accessory drive gear just as they are. This might have a detrimental effect on the durability of accessory members or auxiliary equipments (called herein as “accessory” or “accessories”) and might cause gear noises due to backlashes-of a gear train including the accessory drive gear, as well as gear noises during the operation of a starter motor.